During the construction of an oil or similar well it is necessary to connect successive lengths of pipe. This is achieved by a threaded connection.
Conventionally, a tool known as a power tong is used for gripping one length of pipe and rotating it relative to the next to make or break the joint. Such power tongs comprise an elongate body having, at one end, a pipe receiving recess and a jaw assembly for gripping and rotating a pipe. The jaw assembly is connected to a source of power, for example a hydraulic motor which, in use, rotates the jaw assembly (and the pipe) relative to the elongate body. The reaction force at the other end (the free end) of the elongate body is constrained by a cable which is connected between the free end of the elongate body and a convenient anchor point. A tension measuring device can conveniently be incorporated in the cable to provide an indication of the torque applied to the joint.
In certain situations, particularly where large diameter pipes and large torques are involved no convenient anchor points are available. Furthermore, cables have been known to snap or break loose under tension causing injury and death to drilling personnel.
In order to meet this problem the prior art provided an apparatus for connecting and disconnecting threaded members which apparatus comprises a power tong having a pipe receiving recess and a jaw assembly for gripping and rotating a first pipe, a backup unit for gripping a second pipe, and means for transmitting reaction forces generated by said power tong to said backup unit.
Heretofore the means has comprised a connection between the free end of the elongate body of the power tong and the free end of the backup unit remote from the second pipe.
In use, the reaction force on the body of the power tong is transmitted through the connection and the backup to the second pipe. This creates a force tending to move the pipe receiving recess and the corresponding pipe receiving recess in the backup unit out of axial alignment, thereby tending to bend the pipe.
Whilst conventional steel pipes can easily withstand the bending forces involved this is not the case for lighter alloy pipes (known as "premium tubes") which are far less robust than conventional steel pipes and which must be handled with greater care. Furthermore, such bending forces cause concentrated stresses where the threads of the pipes contact one another and can cause galling, particularly with "premium tubes".
Whilst it would be possible to increase the rigidity of the prior art apparatus this would increase the weight. However, since the apparatus should be easy to handle it is desirable that it should be relatively light.
The aim of at least preferred embodiments of the present invention is to provide an apparatus which is relatively light and which can be used with premium tubes.